Waste implications from minor impurities in European DEMO materials

Abstract

Waste-production predictions for the future demonstration fusion power plant (DEMO) are necessary to produce an accurate picture of the likely environmental and economic costs of radioactive waste disposal at end-of-life. An integrated simulation process combining Monte-Carlo neutron transport simulations, inventory calculations, and extensive and reproducible post-processing algorithms has been used for the evolving European DEMO designs to quantify the time-varying mass inventories in different waste classes for individual regions and components of the reactor vessel, as well as for the reactor as a whole. Waste categories based on UK and French regulations reveal that minor impurities contained in structural steels, particularly Eurofer, as well as in functional materials such as tungsten, and beryllium, can have a significant impact on their waste classification prospects. Predictions for current European DEMO concepts suggest that there may be an issue in disposing of fusion structural-steel waste as low-level waste (LLW) in near-surface repositories. Detailed analysis of the subtleties of these predictions, particularly with regard to the production of long-lived radionuclides such as 14C and 94Nb, reveal that the threshold for acceptance as LLW is only just exceeded in some situations. Several mitigation approaches are discussed in this context. The computational framework developed for these assessments can be rapidly and continuously applied to the maturing DEMO design, helping to guide design choices to mitigate long-lived waste production and ensure that most waste becomes LLW or better within a few decades.

High-Entropy Alloys for Advanced Nuclear Applications

“…The particular activation that an element will undergo depends on the neutron energy spectrum and flux it faces (and hence its place in a reactor relative to the neutron source). Figure 2 , produced by Gilbert et al [ 15 ] using the FISPACT-II neutronics code, indicates how long elements would take to decay to be classed as low level waste in the UK following exposure to the fusion neutron spectrum in the divertor of a hypothetical fusion reactor. This gives a good indication as to activation inside a fusion reactor.…”

“…It has been recognised that unavoidable impurities in alloying elements may mean that guaranteeing low activation in alloys, including HEAs, is difficult. Even a low-activation steel like EUROFER97 is exceptionally tricky to produce without introducing traces of highly-activating elements like Nb, which are derived from the primary ores and are difficult to remove during manufacture [ 15 , 156 ]. One might imagine that in HEAs, with high concentrations of many different elements each with their own impurities, the problem is likely to be exacerbated.…”

“…The component considered is the divertor over a time period equivalent to 5 years of pulsed operation. Reproduced from Gilbert et al [ 15 ].…”

Re-examining the role of nuclear fusion in a renewables-based energy mix

“…EUROFER97 reduced-activation steel is the leading fusion structural material that was designed by the Eurofusion effort [73] and has been chosen as the neutron-facing structural material in the EU-DEMO1 reactor design [10]. However, the latest research using the EU-DEMO1 design, by Gilbert et al (2019) [74] and [75], suggests that the EUROFER97 steel (used for the first wall, breeder blanket and divertor components) will always exceed the reduced-activation criterion, and hence be classed as intermediatelevel waste (ILW).…”

“…Furthermore, Gilbert et al [74] indicate that any beryllium used in a Helium-Cooled Pebble Bed (HCPB) breeder blanket could exceed the reduced-activation criterion due to natural uranium impurities (which activate to become 239 Pu and 241 Am in trace amounts, which have extremely long half-lives).…”